In one way, It depends on the number of moles of any antimicrobial that can inhibit the pathogen. The lower the inhibitory concentration, more the efficacy. We can find out from its concentration used.. ug/ ml or mg/ml. And what really matters is differentiating static or cidal activity of any antimicrobial & bactericidal concentration, rather than just zone of inhibition, which is too preliminary to decide upon efficacy of antimicrobials.

Absolutely yes.

No. Several factors should be taken into consideration after confirming their efficacy through AST. For instance; the physiological status of the patient (pregnant or lactating women), Gender, Age, site of infection (e,g., prostate, CSF, Blood, Urinary tract). Weather the drug is reserved in your country or not. The price of antibiotic is a factor for choice.@ Kukwah Anthony Tufon

I agree with almost all the answers, but there is something very important that nowbody mentioned. The zone diameter arround the different antimicrobials depends on the characteristics of the antimicrobial itself, as for example the molecula hydrosolubility, its size,... These factors make that some antimicrobials spread better than others in the medium

No! The zone of inhibition does not exactly determine the efficacy in vivo with respect to the other drug. Cave, how they diffuse in a culture medium. Besides antimicrobial resistance alertness, the way they are metabolised by the body will also orient me which one I would go for. In your situation, I would still go for nitrofurantoin because it is easily filtered out of blood into urine and thus will be potentially better against urinary tract infections than Amikacin

Of course you are right , PK/PD parameters are very important and I agree that the best option is nitrofurantoin, because it concentrates in urine, and of course because we have to make a prudent use of antimicrobials in our fight against antimicrobial resistance